Assessment of changes in liver blood flow after food intake--comparison of ICG clearance and echo-Doppler

Integrated pharmacokinetics and pharmacodynamics of epoprostenol in healthy subjects

AIM

The aim of the study was to report the first thorough characterization of the pharmacokinetics (PK) and pharmacodynamics (PD) of epoprostenol in an integrated manner.

METHOD

Twenty healthy male subjects received two formulations of i.v. epoprostenol, in a crossover design, in sequential infusions of 2, 4, 6 and 8 ng kg(-1)  min(-1) for 2 h each. A sensitive assay was developed which allowed accurate PK characterization of epoprostenol via analysis of the concentration-time profiles of its two primary metabolites, 6-keto-prostacyclin F(1α) and 6,15-diketo-13,14-dihydro-prostacyclin F(1α) . PD parameters included cardiac output (CO), cardiac index (CIn) and heart rate (HR).

RESULTS

The pharmacokinetics of epoprostenol deviated slightly from dose-proportionality, probably due to a food effect. After infusion of the two formulations of epoprostenol, the t(1/2) values expressed as geometric mean (95% confidence interval) were 0.25 h (0.14, 0.46) and 0.22 h (0.13, 0.38) for 6-keto-prostacyclin F(1α) , and 0.32 h (0.22, 0.45) and 0.34 h (0.26, 0.46) for 6,15-diketo-13,14-dihydro-prostacyclin F(1α) . A single compartment infusion model with first order elimination adequately described the PK of 6-keto-prostacyclin F(1α) . This model also characterized the food effect. Stepwise infusions with epoprostenol resulted in a progressive increase in CO, CIn and HR.

CONCLUSION

Of the two metabolites analyzed, the appearance of 6-keto-prostacyclin F(1α) in plasma was more closely associated with the haemodynamic effects of i.v. epoprostenol. PK and PD profiles showed that CIn relates proportionally and linearly to the plasma concentrations of 6-keto-prostacyclin F(1α) . These results suggest that 6-keto-prostacyclin F(1α) is a suitable surrogate marker of plasma concentrations of epoprostenol.

Pilot study on the influence of liver blood flow and cardiac output on the clearance of propofol in critically ill patients

OBJECTIVE

To investigate the effect of cardiac output and liver blood flow on propofol concentrations in critically ill patients in the intensive care unit.

METHODS

Five medical/surgical critically ill patients were enrolled in this preliminary study. Liver blood flow was measured using sorbitol. The cardiac output was measured by bolus thermodilution. NONMEM ver. V was applied for propofol pharmacokinetic analysis.

RESULTS

The clearance of propofol was positively influenced by the liver blood flow (P < 0.005), whereas no significant correlation between cardiac output and propofol clearance was found. A correlation between liver blood flow and cardiac output or cardiac index could not be assumed in this patient group.

CONCLUSIONS

Liver blood flow is a more predictive indicator than cardiac output for propofol clearance in critically ill patients when the techniques of hepatic sorbitol clearance and bolus thermodilution, respectively, are used. Further study is needed to determine the role played by liver blood flow and cardiac output on the pharmacokinetics of highly extracted drugs in order to reduce the observed high interindividual variabilities in response in critically ill patients.

Changes in regional blood volume and blood flow during static handgrip

Increased blood pressure (BP) and heart rate during exercise characterizes the exercise pressor reflex. When evoked by static handgrip, mechanoreceptors and metaboreceptors produce regional changes in blood volume and blood flow, which are incompletely characterized in humans. We studied 16 healthy subjects aged 20-27 yr using segmental impedance plethysmography validated against dye dilution and venous occlusion plethysmography to noninvasively measure changes in regional blood volumes and blood flows. Static handgrip while in supine position was performed for 2 min without postexercise ischemia. Measurements of heart rate and BP variability and coherence analyses were used to examine baroreflex-mediated autonomic effects. During handgrip exercise, systolic BP increased from 120 +/- 10 to 148 +/- 14 mmHg, whereas heart rate increased from 60 +/- 8 to 82 +/- 12 beats/min. Heart rate variability decreased, whereas BP variability increased, and transfer function amplitude was reduced from 18 +/- 2 to 8 +/- 2 ms/mmHg at low frequencies of approximately 0.1 Hz. This was associated with marked reduction of coherence between BP and heart rate (from 0.76 +/- 0.10 to 0.26 +/- 0.05) indicative of uncoupling of heart rate regulation by the baroreflex. Cardiac output increased by approximately 18% with a 4.5% increase in central blood volume and an 8.5% increase in total peripheral resistance, suggesting increased cardiac preload and contractility. Splanchnic blood volume decreased reciprocally with smaller decreases in pelvic and leg volumes, increased splanchnic, pelvic and calf peripheral resistance, and evidence for splanchnic venoconstriction. We conclude that the exercise pressor reflex is associated with reduced baroreflex cardiovagal regulation and driven by increased cardiac output related to enhanced preload, cardiac contractility, and splanchnic blood mobilization.

Persistent splanchnic hyperemia during upright tilt in postural tachycardia syndrome

Previous investigations have allowed for stratification of patients with postural tachycardia syndrome (POTS) on the basis of peripheral blood flow. One such subset, comprising "normal-flow POTS" patients, is characterized by normal peripheral resistance and blood volume in the supine position but thoracic hypovolemia and splanchnic blood pooling in the upright position. We studied 32 consecutive 14- to 22-yr-old POTS patients comprising 13 with low-flow POTS, 14 with normal-flow POTS, and 5 with high-flow POTS and 12 comparably aged healthy volunteers. We measured changes in impedance plethysmographic (IPG) indexes of blood volume and blood flow within thoracic, splanchnic, pelvic (upper leg), and lower leg regional circulations in the supine posture and during incremental tilt to 20 degrees, 35 degrees, and 70 degrees. We validated IPG measures of thoracic and splanchnic blood flow against indocyanine green dye-dilution measurements. We validated IPG leg blood flow against venous occlusion plethysmography. Control subjects developed progressive vasoconstriction with incremental tilt. Splanchnic blood flow was increased in the supine position in normal-flow POTS, despite marked peripheral vasoconstriction, and did not change during incremental tilt, producing progressive splanchnic hypervolemia. Absolute hypovolemia was present in low-flow POTS, all supine flows and volumes were reduced, there was no vasoconstriction with tilt in all segments, and segmental volumes tended to increase uniformly throughout tilt. Lower body (pelvic and leg) flows were increased in high-flow POTS at all angles, with consequent lower body hypervolemia during tilt. Our main finding is selective and maintained orthostatic splanchnic vasodilation in normal-flow POTS, despite marked peripheral vasoconstriction in these same patients. Local splanchnic vasoregulatory factors may counteract vasoconstriction and venoconstriction in these patients. Lower body vasoconstriction in high-flow POTS was abnormal, and vasoconstriction in low-flow POTS was sustained at initially elevated supine levels.

Influence of enteral and parenteral nutrition on splanchnic hemodynamics in septic patients

BACKGROUND

Enteral nutrition is believed to augment splanchnic perfusion, thereby preserving splanchnic integrity, whereas parenteral nutrition does not offer this benefit. In an effort to study this, we compared splanchnic oxygen exchange and blood flow in critically ill, septic patients to normal controls during enteral or total parenteral nutrition.

METHODS

Splanchnic oxygen exchange and hepatic blood flow characteristics in 14 critically ill, septic patients were compared to 19 normal controls while fasting and during nutrient administration. Nutrients were delivered as intraduodenal feedings or parenteral nutrition. Splanchnic hemodynamics were measured at baseline, 90 min, and 210 min during nutrient administration. Hepatic blood flow index (HBFI) by indocyanine green dye (ICG) clearance, splanchnic oxygen consumption index (SplVO(2)I), and hepatic venous oxygen saturation (ShvO(2)) were measured using hepatic venous catheterization. Plasma volume (PV) was measured from the volume of ICG distribution. Results were analyzed using population means (+/-SD) and one-way analysis of variance.

RESULTS

There was no statistical change in HBFI, SplVO(2)I, PV or ShvO(2) over the study time interval within any group (p < 0.05), irrespective of whether enteral or parenteral nutrition was the nutrient source. Septic patients, whether fasting or receiving nutrition, demonstrated higher HBFI and SplVO(2)I levels, whereas ShvO(2) levels were uniformly lower throughout the study compared to normal controls.

CONCLUSIONS

Critically ill patients exhibited a hyperdynamic splanchnic state as indicated by the marked increase in HBFI and SplVO(2)I. However, neither nutrient regimen at clinically relevant rates altered splanchnic hemodynamics over the course of study. Thus, enteral nutrients do not appear to offer hemodynamic protection to the splanchnic system in critically ill patients.

Modeling of hepatic elimination and organ distribution kinetics with the extended convection-dispersion model

The conventional convection-dispersion (also called axial dispersion) model is widely used to interrelate hepatic availability (F) and clearance (Cl) with the morphology and physiology of the liver and to predict effects such as changes in liver blood flow on F and Cl. An extended form of the convection-dispersion model has been developed to adequately describe the outflow concentration-time profiles for vascular markers at both short and long times after bolus injections into perfused livers. The model, based on flux concentration and a convolution of catheters and large vessels, assumes that solute elimination in hepatocytes follows either fast distribution into or radial diffusion in hepatocytes. The model includes a secondary vascular compartment, postulated to be interconnecting sinusoids. Analysis of the mean hepatic transit time (MTT) and normalized variance (CV2) of solutes with extraction showed that the discrepancy between the predictions of MTT and CV2 for the extended and unweighted conventional convection-dispersion models decreases as hepatic extraction increases. A correspondence of more than 95% in F and Cl exists for all solute extractions. In addition, the analysis showed that the outflow concentration-time profiles for both the extended and conventional models are essentially identical irrespective of the magnitude of rate constants representing permeability, volume, and clearance parameters, providing that there is significant hepatic extraction. In conclusion, the application of a newly developed extended convection-dispersion model has shown that the unweighted conventional convection-dispersion model can be used to describe the disposition of extracted solutes and, in particular, to estimate hepatic availability and clearance in both experimental and clinical situations.

Effect of enteral versus parenteral feeding on hepatic blood flow and steady state propofol pharmacokinetics in ICU patients

OBJECTIVE

The main objective of this study was to evaluate the effect of switching from parenteral to enteral feeding on liver blood flow and propofol steady-state blood concentrations in patients in the intensive care unit (ICU).

DESIGN AND PATIENTS

Steady-state blood concentrations of propofol were measured in eight ICU patients before (on days D -3, D -2, and D -1) and after (on days D + 1, D + 2, and D + 3) switching from parenteral to enteral feeding (on day DO). All patients received a continuous intravenous infusion of propofol (4.5 mg x kg(-1) x h(-1)) from several days before the start of the study, continuing throughout the experimental period. Hepatic blood flow was estimated by measuring steady-state D-sorbitol hepatic clearance.

RESULTS

Hepatic blood flow was high and was not affected by switching from parenteral to enteral feeding: 33 +/- 8 ml x min(-1) x kg(-1) (mean +/- SD) and 33 +/- 10 ml min(-1) x kg(-1) on D -3 and D -1, respectively, as compared to 37 +/- 11 ml x min(-1) kg(-1) and 34 +/- 8 ml x min(-1) x kg(-1) on days D + 1 and D + 3, respectively. Systemic clearance of propofol was much higher than liver blood flow with average values on the six observation days ranging from 74.0 to 81.2 ml x min(-1) x kg(-1) and was not affected by switching from parenteral to enteral feeding.

CONCLUSIONS

Liver blood flow and systemic clearance of propofol were not affected by switching from parenteral to enteral feeding in the eight ICU patients studied. Extrahepatic clearance accounted for at least two thirds of the overall systemic clearance of propofol.

The influence of nifedipine and captopril on liver blood flow in healthy subjects

AIMS

Application of single methods to assess liver blood flow (LBF) yielded conflicting results on the magnitude and duration of effect on LBF of oral nifedipine and captopril. The aim of this study was to investigate the influence of these drugs on LBF by simultaneous use of ICG infusion and echo-Doppler.

METHODS

The study was performed according to a double-blind, placebo-controlled, randomized, cross-over design in nine healthy male volunteers. After an overnight fast and an equilibration period, subjects received a continuous i.v. indocyanine green (ICG) infusion for 4 h. At presumed ICG steady state (t=45 min), subjects were dosed with oral nifedipine (20 mg), captopril (50 mg) or placebo. During the experiment, blood sampling for ICG assay and measurement of portal venous blood flow (echo-Doppler) took place regularly. Treatments were compared using analysis of variance. Differences are reported with 95% confidence intervals (CI).

RESULTS

The area under the curves (AUC) for ICG over 1 h and over 3 h after nifedipine were 15% (difference in AUC: + 0.6, + 7.0 mg l(-1) min) and 22% (+ 7.0, + 28.4 mg l(-1) min) lower compared with placebo. After captopril, the AUC values were 8-10% lower compared with placebo but the 95% CIs included zero. Portal venous flow was 15% (+ 5, + 86 ml min(-1)) higher compared to placebo after nifedipine but not after captopril (-3%; -49, +33 ml min(-1)). The duration of effect on liver blood flow lasted approximately 2 h but was variable (range: 40-160 min). The time to maximal blood flow increase and the duration of effect after nifedipine were very similar for both measures of LBF. Changes in ICG concentrations could be reasonably well predicted from the changes in portal blood flow.

CONCLUSIONS

Nifedipine increases LBF for a substantial period of time but the effect is variable between subjects. This effect could be detected by both the ICG method and echo-Doppler and the findings of both methods were in agreement. In this respect it is likely that captopril does not influence LBF in healthy volunteers as no effect was detected with either method.